U.S. patent number 5,587,003 [Application Number 08/407,817] was granted by the patent office on 1996-12-24 for removal of carbon dioxide from gas streams.
This patent grant is currently assigned to The BOC Group, Inc.. Invention is credited to Martin Bulow, Loc Dao, Frank R. Fitch.
United States Patent |
5,587,003 |
Bulow , et al. |
December 24, 1996 |
Removal of carbon dioxide from gas streams
Abstract
Carbon dioxide is removed from a gas stream by passing the gas
stream through a bed of natural or synthetic clinoptilolite or
their chemically-modified derivatives. The process is particularly
advantageous when applied to the removal of ppm levels of carbon
dioxide from gas streams at temperatures above 20.degree. C.
Inventors: |
Bulow; Martin (Basking Ridge,
NJ), Dao; Loc (Bound Brook, NJ), Fitch; Frank R.
(Bedminster, NJ) |
Assignee: |
The BOC Group, Inc. (New
Providence, NJ)
|
Family
ID: |
23613632 |
Appl.
No.: |
08/407,817 |
Filed: |
March 21, 1995 |
Current U.S.
Class: |
95/123; 95/139;
95/902 |
Current CPC
Class: |
B01D
53/04 (20130101); Y02C 10/06 (20130101); B01D
2256/24 (20130101); Y02C 10/08 (20130101); Y02C
20/40 (20200801); B01D 2259/416 (20130101); B01D
53/02 (20130101); B01D 2256/18 (20130101); B01D
2256/10 (20130101); Y10S 95/902 (20130101); B01D
2257/504 (20130101); Y02C 10/04 (20130101); B01D
2256/12 (20130101); B01D 2253/108 (20130101) |
Current International
Class: |
B01D
53/04 (20060101); B01D 053/047 () |
Field of
Search: |
;62/13,18
;95/117-120,123,139,902 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
|
|
61-255995 |
|
Nov 1986 |
|
JP |
|
0552102 |
|
Apr 1977 |
|
SU |
|
0679228 |
|
Aug 1979 |
|
SU |
|
Other References
Mark W. Ackley and Ralp T. Yang--Diffusion in Ion-Exchanged
Clinoptitolites--AlChE Journal--Nov. 1991 vol. 37, No. 11. .
Mark W. Ackley, R. F. Giese and Ralph T. Yan--Zeolites 1992, vol.
12, Sep./Oct. .
Natural Zeolites--Academy of Sciences of the Georgian
SSR--Scientific Council on Adsorbents of the Soviet Union Academy
of Sciences--P. G. Melikishvili--Oct. 29-31, 1976..
|
Primary Examiner: Spitzer; Robert
Attorney, Agent or Firm: Reap; Coleman R. Cassett; Larry
R.
Claims
What is claimed is:
1. A method of removing carbon dioxide from a gas stream selected
from oxygen, nitrogen, argon and mixtures of these which contains
carbon dioxide at a partial pressure not greater than about 10 mbar
as an impurity, comprising subjecting said gas stream to adsorption
using clinoptilolite as the adsorbent at a temperature in the range
of about 20.degree. to about 80.degree. C., thereby adsorbing
substantially all of the carbon dioxide from the gas stream.
2. The method of claim 1, wherein the adsorption is part of a
process selected from temperature swing adsorption, pressure swing
adsorption, vacuum swing adsorption and combinations of these.
3. The method of claim 2, wherein said clinoptilolite has
exchangeable cations selected from ions of Group 1A, Group 2A,
Group 3A, Group 3B, the lanthanide series and mixtures of
these.
4. The method of claim 1 or claim 2, wherein said clinoptilolite is
selected from natural clinoptilolite, synthetic clinoptilolite,
sodium-exchanged clinoptilolite, potassium-exchanged
clinoptilolite, lithium-exchanged clinoptilolite, calcium-exchanged
clinoptilolite magnesium-exchanged clinoptilolite, barium-exchanged
clinoptilolite, and mixtures of these.
5. The method of claim 1 or claim 2, wherein the adsorption is
carried out at a temperature in the range of about 30.degree. to
about 60.degree. C.
6. The method of claim 5 wherein said gas stream is air.
7. The method of claim 1 or claim 2, additionally comprising, prior
to removing carbon dioxide from said gas stream, removing water
vapor from the gas stream by passing the gas stream through an
adsorbent selected from alumina, silica gel, and mixtures of
these.
8. A method of separating air comprising the steps:
(a) prepurifying air containing carbon dioxide at a partial
pressure not greater than about 10 mbar by subjecting the air to a
temperature swing adsorption process comprising an adsorption step
carried out at a temperature in the range of about 20.degree. to
about 80.degree. C. and an adsorbent regeneration step using
clinoptilolite as adsorbent, thereby adsorbing substantially all of
the carbon dioxide from the air; and
(b) subjecting the prepurified air to cryogenic distillation,
thereby producing high purity nitrogen, high purity oxygen or both
of these.
9. The method of claim 8, wherein said adsorbent additionally
adsorbs water vapor from said air.
10. The method of claim 8, additionally comprising, prior to step
(a), the step of removing water vapor from the air by passing the
air through an adsorbent selected from alumina, silica gel and
mixtures of these.
11. The method of claim 8, wherein the concentration of carbon
dioxide in said air is not greater than about 500 ppm.
12. The method of claim 8, wherein said clinoptilolite is selected
from natural clinoptilolite, lithium-exchanged clinoptilolite,
calcium-exchanged clinoptilolite and mixtures of these.
13. The method of claim 12, wherein said adsorption step is carried
out at a temperature in the range of about 30.degree. to about
60.degree. C.
Description
FIELD OF THE INVENTION
This invention relates to the removal of carbon dioxide from gas
streams, and more particularly to the prepurification of air by the
removal of carbon dioxide from air prior to air separation.
BACKGROUND OF THE INVENTION
Gases that occur in nature or which are produced in industrial
processes often contain carbon dioxide in small amounts. For
example atmospheric air generally contains about 300 or more parts
per million (ppm) carbon dioxide. Because of certain process
constraints or a particular end use that the gas is intended for,
it may sometimes be desirable or necessary to remove the carbon
dioxide from the gas. For example, air that is separated into
various component products by cryogenic distillation (cryogenic air
separation) must be substantially free of both carbon dioxide and
moisture. Cryogenic air separation is carried out at temperatures
well below the freezing point of carbon dioxide and water.
Consequently, if these components are not removed prior to cooling
of the air they will freeze in and eventually clog the air
separation process equipment.
Small amounts of carbon dioxide and moisture are removed from gas
streams by various techniques, such as condensation, reversing heat
exchange freezing and adsorption. A particularly preferred method
is adsorption using an adsorbent which adsorbs carbon dioxide (and
water vapor) more strongly than it adsorbs other components of the
gas stream. For example, it is common to remove carbon dioxide from
an air stream that is to be cryogenically separated, by passing the
gas stream through a bed of zeolite 13X. U.S. Pat. No. 3,885,927,
issued to Sherman et al. on May 27, 1975, discloses the use of type
X zeolite containing at least 90 equivalent percent barium cations
for the removal of carbon dioxide from gas streams containing not
more than 1000 ppm carbon dioxide, at temperatures of -40.degree.
to 120.degree. F. U.S. Pat. No. 4,775,396, issued to Rastelli et
al. on Oct. 4, 1988, discloses the adsorption of carbon dioxide
from gas streams by pressure swing adsorption at temperatures of
-50.degree. to 100.degree. C., the adsorbent being a zeolite having
a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of from 2 to 100 and
containing at least 20 equivalent percent of one or more cations
selected from zinc, rare earth, hydrogen and ammonium cations and
not more than 80 equivalent percent of alkali metal or alkaline
earth metal cations.
Zeolite 13X efficiently removes small amounts of carbon dioxide
(and water vapor) from air streams at low temperatures, i.e.
temperatures of about 5.degree. C. or lower, because it more
strongly adsorbs these components than it adsorbs nitrogen, oxygen
or argon. However, the carbon dioxide selectivity, and, to a
greater extent, the adsorption capacity of zeolite 13X diminish
rapidly as the temperature of the gas being separated increases,
and the separation process becomes infeasible at temperatures above
about 20.degree. C. Since ambient temperatures are often above the
preferred 5.degree. C. adsorption temperature, and since, because
of the heat of adsorption, there is a tendency for adsorption bed
temperatures to increase considerably during the course of an
adsorption process, it is usually necessary to cool air fed to an
adsorption-based air prepurification plant by means of external
refrigeration to maintain the gas at temperatures below 20.degree.
C. This reduces the overall efficiency of the air separation
process, since energy must be consumed to provide the necessary
refrigeration.
It would be very advantageous to completely eliminate the need for
refrigeration or to significantly reduce the amount of
refrigeration required in commercial air separation
adsorption-based prepurification procedures, since that would
enhance the overall economic attractiveness of the air separation
process. The present invention provides a novel carbon dioxide
adsorption process which provides such an advantage, and eliminates
the need for environmentally harmful refrigerants, such as the
freons.
SUMMARY OF THE INVENTION
According to the invention, a gas stream is purified by the removal
of carbon dioxide from the gas stream by passing the gas stream
through a bed of clinoptilolite at a temperature in the range of
about -50.degree. to about 80.degree. C. The process of the
invention can be used to purify any gas that is less strongly
adsorbed by clinoptilolite than carbon dioxide and which contains
not more than about 1000 parts per million (ppm) levels of carbon
dioxide as an impurity. Typical of gases that can be purified by
the process of the invention are air, nitrogen, oxygen, argon,
methane, etc.
The adsorbent may be natural clinoptilolite, or it may be
cation-exchanged with one or more of the various monovalent,
divalent or trivalent ions selected from Groups IA, IIA and IIIA of
the Periodic Table, lanthanide series ions, chromium (III), iron
(III), zinc (II) or copper (II). Preferred adsorbents are
clinoptilolite having as exchangeable cations one or more of
sodium, potassium, lithium, calcium, magnesium, barium, strontium,
aluminum, scandium, gallium, indium, yttrium, lanthanum, cerium,
praseodymium and neodymium ions. The most preferred cations are
sodium, lithium, calcium, magnesium, aluminum, cerium and lanthanum
and mixtures of these.
The adsorption step of the process of the invention is beneficially
carried out at temperatures in the range of about 20.degree. to
about 80.degree. C. Very good results are obtained when the
adsorption step is carried out at a temperature in the range of
about 30.degree. to about 60.degree. C.
The carbon dioxide purification is preferably carried out by a
cyclic process, more preferable as pressure swing adsorption (PSA),
temperature swing adsorption (TSA), or combinations of these. In
the most preferred embodiment, the process is a TSA process.
The carbon dioxide concentration of gas streams purified by the
process of the invention is preferable not more than 600 ppm, and
most preferably not more than 350 ppm.
The process of the invention can comprise the single operation of
carbon dioxide adsorption, or it may comprise a combination of
purification operations, including carbon dioxide adsorption and
one or more of air separation, hydrogen oxidation, carbon monoxide
oxidation, etc. In a preferred procedure carbon dioxide is removed
from air by the above-described adsorption method and the purified
air is separated by cryogenic distillation into nitrogen, oxygen,
argon or combinations of two or more of these.
The carbon dioxide adsorption step with the clinoptilolite
adsorbent can also be used to remove moisture from the gas stream,
if present. In a preferred embodiment, moisture is removed prior to
carbon dioxide adsorption by, for example, passing the gas stream
through a desiccant, preferably alumina, silica gel or mixtures of
these.
DETAILED DESCRIPTION OF THE INVENTION
The process of the invention is particularly useful for removing
carbon dioxide at low concentrations i.e. parts per million (ppm)
levels, from gas streams at temperatures above about 20.degree. C.
Although the process can be successfully used to remove carbon
dioxide from gas streams which contain the carbon dioxide at
concentrations greater than 1000 ppm, it is most effective for
removing carbon dioxide from a gas stream when the carbon dioxide
is present in the gas stream at concentrations up to about 1000
parts per million (ppm).
The adsorbents useful in the process of the invention are the
natural and synthetic clinoptilolites and their chemically modified
derivatives. Natural clinoptilolite generally has as exchangeable
cations one or more of potassiumions, sodiumions, calciumions and
magnesiumions. Cation-exchanged natural or synthetic clinoptilolite
may also be used in the invention. Included among the ions that may
occupy exchangeable cation sites on the clinoptilolite adsorbent
are ions of Groups IA, IIA, IIIA, IIIB of the periodic table,
trivalent ions of the lanthanide series of elements, zinc (II)ion,
copper (II)ion, chromium (III)ion, iron (III)ion, the ammonium ion,
the hydronium ion or mixtures of two or more ions from any of these
categories. Preferred Group IA ions are sodium, potassium and
lithium ions; preferred Group IIA ions are magnesium, calcium,
strontium and barium ions; preferred Group IIIA and IIIB ions are
aluminum, scandium, gallium, indium and yttrium; and preferred
trivalent lanthanide ions are lanthanum, cerium, praseodymium and
neodymium. The most preferred clinoptilolites are the natural and
synthetic clinoptilolites having as exchangeable cations one or
more ions selected from: sodium, potassium, lithium, calcium,
magnesium and barium ions.
The process of the invention may be carried out in a single
adsorption vessel or a battery of two or more beds arranged in
parallel and adapted to be operated in a cyclic process comprising
adsorption and desorption. In such systems the beds are cycled out
of phase to assure a pseudo-continuous flow of purified gas from
the adsorption system.
The process of the invention is generally practiced as a cyclical
process, such as temperature swing adsorption, pressure swing
adsorption, vacuum swing adsorption, or combinations of these. The
process is particularly useful for removing small amounts of carbon
dioxide from air by temperature swing adsorption. The carbon
dioxide removal process is ideally coupled with an air separation
process, such as cryogenic distillation of air, to produce high
purity nitrogen, oxygen, argon or combinations of two or more of
these high purity gas products.
The temperature at which the adsorption step is carried out may
vary from a minimum temperature of about -50.degree. C. to a
maximum of about 80.degree. C. It has been discovered that the
efficiency of the adsorption process does not drop off with
increasing adsorption temperature, as rapidly as when conventional
adsorbents are used in the process. This feature makes the process
advantageous for use in warm weather climates where the temperature
during the adsorption step is above about 20.degree. C., or even
above about 40.degree. C. Although the adsorption process can be
carried out at temperatures up to about 80.degree. C., it is
preferable that the temperature not exceed about 60.degree. C. and
most preferable that it not exceed about 50.degree. C.
The absolute pressures at which the adsorption step is carried out
generally ranges from about 0.2 to about 20 bar, and preferably
ranges from about 1 to 10 bar.
When the adsorption process is PSA the regeneration step is
generally carried out a temperature in the neighborhood of the
temperature at which the adsorption step is carried out and at an
absolute pressure lower than the adsorption pressure. The pressure
during the regeneration step of PSA cycles is usually in the range
of about 200 to about 5000 millibar, and preferably in the range of
about 100 to about 2000 millibar. When the adsorption process is
TSA, bed regeneration is carried out at a temperature higher than
the adsorption temperature, usually in the range of about
50.degree. to about 250.degree. C., and preferably in the range of
about 100 to 200.degree. C. In the TSA embodiment, the pressure can
be the same during the adsorption and regeneration steps, but it is
often preferred to desorb to about atmospheric pressure. When a
combination PSA/TSA process is employed, the temperature and
pressure during the bed regeneration step are higher and lower,
respectively, than they are during the adsorption step.
In starting a cyclical process according to the invention, the
gaseous feed stream from which carbon dioxide is to be removed is
introduced into an adsorption vessel containing a bed of the
above-mentioned adsorbent. As the gas passes through the bed of
adsorbent carbon dioxide is adsorbed and a substantially carbon
dioxide-free nonadsorbed product gas passes out of the adsorption
vessel through the nonadsorbed gas outlet. As the adsorption step
proceeds, a carbon dioxide front forms in the adsorbent bed and
slowly moves toward the nonadsorbed gas outlet end of the bed. When
the adsorbed carbon dioxide front traveling through the adsorption
vessel(s) in which the adsorption step is being carried out reaches
the desired point in the vessel(s), the adsorption process in these
vessel(s) is terminated and these vessel(s) enter the regeneration
mode. During regeneration, the carbon dioxide-loaded vessels are
depressurized, if the adsorption cycle is pressure swing
adsorption; heated, if a temperature swing adsorption cycle is
employed; or both depressurized and heated, if a combination
pressure swing-temperature swing process is used.
The method of regeneration of the adsorption beds depends upon the
type of adsorption process employed. In the case of pressure swing
adsorption, the regeneration phase generally includes a
countercurrent depressurization step during which the beds are
vented countercurrently until they attain the desired lower
pressure. If desired the pressure in the beds may be reduced to
subatmospheric pressure by means of a vacuum inducing device, such
as a vacuum pump.
In some cases, in addition to the countercurrent depressurization
step(s), it may be desirable to countercurrently purge the bed with
the nonadsorbed product gas stream exiting the adsorbent bed(s). In
this case the bed(s) may be countercurrently purged with
nonadsorbed gas, and the purge step is usually initiated towards
the end of the countercurrent depressurization step, or subsequent
thereto. During this purge step, the purge gas can be introduced
into the adsorbent bed from an intermediate storage facility when
the adsorption system comprises a single adsorber; or from another
adsorber that is in the adsorption phase, when the adsorption
system comprises multiple adsorbers arranged in parallel and
operated out of phase.
The adsorption cycle may contain steps other than the fundamental
steps of adsorption and regeneration. For example, it may be
advantageous to depressurize the adsorption bed in multiple steps,
with the first depressurization product being used to partially
pressurize another bed in the adsorption system. This will further
reduce the amount of gaseous impurities in the nonadsorbed product
gas.
According to a preferred embodiment of the invention, a gas stream,
such as air, is introduced into an adsorption vessel containing a
clinoptilolite of the type described above. The gas stream may be
at a temperature as low as -50.degree. C., or less, or as high as
80.degree. C. Provided that the concentration of carbon dioxide in
the gas stream is not significantly greater than about 1000 ppm,
substantially all of the carbon dioxide will be removed from the
gas stream, and the substantially carbon dioxide-free product gas
will issue from the nonadsorbed product gas outlet of the
adsorption vessel. When the carbon dioxide adsorption front reaches
a predetermined point in the adsorption vessel, usually near the
nonadsorbed product gas outlet, the adsorption process in the
vessel is terminated, and the adsorbent bed contained in the vessel
is regenerated in one of the methods described above. If the
adsorption plant is a multiple bed system, adsorption will
immediately begin in a second bed, so that the continuity of the
purification process will not be interrupted. The prepurified gas
can be subjected to further processing. For example, in cryogenic
air separation operations, the prepurified air is sent to a
cryogenic distillation (or adsorption) plant for fractionation into
one or more high purity gases. If desired, a waste gas stream from
the air separation plant can be recycled to the prepurification
plant for use as purge gas during bed regeneration.
It will be appreciated that it is within the scope of the present
invention to utilize conventional equipment to monitor and
automatically regulate the flow of gases within the system so that
it can be fully automated to run continuously in an efficient
manner.
The invention is further illustrated by the following example in
which, unless otherwise indicated, parts, percentages and ratios
are on a volume basis.
EXAMPLE 1
Equilibrium adsorption isotherms for carbon dioxide were measured
using a Cahn microbalance at a series of pressures in the range of
2 to 300 mbar at temperatures of 5.degree. C., 35.degree. C. and
50.degree. C. for a conventional sodium X zeolite (NaX) having
silicon-to-aluminum atomic ratio of 1.25, and for an Indonesian
natural clinoptilolite which was first beneficiated by washing with
hot water, and then extensively ion-exchanged with calcium chloride
solution of 80.degree. C. Chemical analysis of the clinoptilolite
sample (Indonesian natural clinoptilolite) showed its weight
percentage composition to be: 64.7% SiO.sub.2 ; 13.8% Al.sub.2
O.sub.3 ; 3.9% CaO; 2.5% K.sub.2 O; 1.0% Fe.sub.2 O.sub.3 ; 0.8%
MgO; 0.26% TiO.sub.2 ; 0.23% Na.sub.2 O; and 0.01% MnO. Each sample
of adsorbent (about 60 mg) was activated by being evacuated at
350.degree. C. for 1.5 hours in situ in the Cahn microbalance
before the first run and between the isotherms taken at each
temperature. Each test was conducted until equilibrium was
achieved, which required up to 3 hours for the lowest partial
pressures of carbon dioxide. The results of the experiments are
recorded in the table.
______________________________________ Pressure., mbar 2 5 10 50
100 300 ______________________________________ Adsorbent Temp.,
Carbon Dioxide Uptake, mmol/gm of .degree.C. adsorbent NaX 5 1.24
1.80 2.23 3.53 4.11 4.79 Ca Clino..sup.1 5 1.13 1.31 1.46 1.89 2.04
2.18 NaX 35 0.45 0.87 1.26 2.26 2.78 3.73 Ca Clino. 35 0.81 1.04
1.17 1.47 1.65 1.93 NaX 50 0.25 0.55 0.87 1.83 2.27 3.14 Ca Clino.
50 0.64 0.91 1.07 1.34 1.48 1.76
______________________________________ .sup.1 calcium
clinoptilolite
From the table it is clear that at moderately high CO.sub.2 partial
pressures (e.g. 300 mbar) the calcium clinoptilolite used in this
example has a much lower CO.sub.2 capacity than does conventional
sodium X adsorbent. The unexpected nature of this invention is
exemplified in the results obtained at less than or equal to 5 mbar
of CO.sub.2, which is typical of air at a pressure of about 15
atmospheres, and at a temperature greater than 20.degree. C. The
capacities of the adsorbents of this invention are more than 20%
greater, and in some cases more than twice those of the
conventional type X adsorbent under the same conditions.
Although the invention has been described with particular reference
to specific equipment arrangements, to specific adsorption cycles,
and to specific experiments, these features are merely exemplary of
the invention and variations are contemplated. For example, the
adsorption cycle may include more than two bed equalization steps,
and the purge step and/or the nonadsorbed product backfill step may
be included or eliminated, as desired. Furthermore, the duration of
the individual steps and the operating conditions may be varied.
The scope of the invention is limited only by the breadth of the
appended claims.
* * * * *